Recently, as a transmission used in an automobile and the like, a metallic-belt-type continuously variable transmission and a toroidal continuously variable transmission have been developed and been already in practical use. In a lubricating oil used in such continuously variable transmissions, it is sought to reduce a viscosity and increase a viscosity index in order to improve an energy-saving property. On the other hand, since a lubricating oil with a low initial viscosity is easily affected by viscosity reduction caused by shearing, a small viscosity reduction caused by shearing is desired.
Accordingly, there has been proposed a lubricating oil having an energy-saving property and a shear stability that are balanced by simultaneously using a base oil having a relatively high viscosity and an shear-resistant viscosity index improver. For instance, Patent Literatures 1 to 3 each disclose a lubricating oil composition in which a small viscosity reduction by shearing is achieved by increasing a viscosity of a base oil and using PMA (polymethacrylate) or OCP (olefin copolymer) having a low molecular weight.